Rotary pipe bending apparatus commonly employs a bend die mounted for rotation about a bend axis, a clamp die mounted for motion toward the bend die to clamp a pipe against the bend die, and a pressure die that is urged toward the bend die to press a rearward portion of the pipe to be bent against the bend die. Machines of this type are illustrated, for example, in U.S. Pat. No. 3,974,676 and several of the patents referred to therein, and in my co-pending applications Ser. No. 692,585, filed June 3, 1976 for Apparatus for Bending Tube and its parent application Ser. No. 614,946, filed Sept. 19, 1975 for Method and Apparatus for Bending Tube, now abandoned. The disclosures of these applications are fully incorporated herein by this reference. In general, in rotary pipe bending machines of this class, a pipe initially positioned at the bend die is clamped against the bend die by driving a clamp die toward the bend die under the force of a clamp die hydraulic cylinder. A pressure die hydraulic cylinder is operated to drive the pressure die toward the pipe and the bend die, and a third hydraulic cylinder, the bending cylinder, is operated to rotate the bend die together with the clamp die around the bend axis. The pipe, clamped between the clamp and bend dies, is pulled and bent around the bend die and a rearward portion of the pipe is restrained by the pressure die. The pressure die is often operated to create friction between the pipe and the pressure die (which presses the pipe against the bend die), in a so-called "wiping" action, such that an axial restraint is exerted on the pipe sufficient to stretch it beyond its yield, to thereby provide a drawing action. Draw bending, which stretches the outside of the pipe bend, is employed to avoid buckling of the pipe wall that may occur in simple compression or pressure bending, where the pipe is bent without such stretching. In many cases the pressure die is driven forwardly, as the pipe is bent around the bend die, by a fourth hydraulic cylinder, often termed a boost cylinder. This may control the amount of stretch. The clamp die presses the pipe against the bend die with a force sufficient to prevent slippage of the pipe relative to the clamp die during the draw forming.
Control of the pressure die is important for proper bending. Drawing, necessarily employed to accomplish many types of bending, is controlled in part by pressure exerted by the pressure die. Forces applied to the pipe during bending vary with the nature of the pipe to be bent, its size and material and other factors. Thus it is common to provide one or more gages to monitor pressures being exerted, such as the force exerted by the pressure die, for example, in an attempt to control the pressure die force relative to the bending force required for a particular bend being made. It is difficult and time-consuming to adjust the various forces to optimum values and, moreover, such optimum values may not always be known or readily available.
In prior rotary bending machines use of three or more separate hydraulic or other kinds of motors has greatly increased complexity and costs, with a consequent decrease in reliability and life of the machines.
Single motors are employed in press bending where a ram is driven against an intermediate portion of a pipe that is restrained at opposite sides of the ram by a pair of wing dies.
The U.S. Pat. No. to Garner et al 3,531,963 describes a type of press bending in which one hydraulic motor is employed to bend a pipe but does not mount auxiliary dies to move toward the bend die, does not drive the dies to press the pipe toward the bend die, and does not suggest draw forming.
The ability to make both right hand and left hand bends generally requires expensive and time-consuming tooling changes. U.S. Pat. No. 3,017,917 to Street shows an attempt to minimize such tooling change time and labor, but with the added burden of a complex 180.degree. rotation of the entire bend head.
Accordingly, it is an object of the present invention to provide improved methods and apparatus for rotary pipe bending that eliminate or minimize above-mentioned problems.